Lighting
Light Bulb Socket Sizes
The number in a bulb base code is its diameter in millimetres. Everything else about the naming is regional habit.
E26 is the standard household socket in North America; E27 is the European one. The E stands for Edison and the number is the thread diameter in millimetres. At IEC 60061 cap tolerances the two do thread together — the real difference is electrical: E26 fixtures run at 120 V, E27 fixtures at 220–240 V. Never put a 120 V-rated bulb into a 240 V fixture.
| Base | Name | Diameter | Typical use | Region |
|---|---|---|---|---|
| E12 | Candelabra | 12 mm | Chandeliers, night lights, decorative fixtures | North America |
| E14 | Small Edison | 14 mm | The European equivalent of E12 — not interchangeable | Europe |
| E17 | Intermediate | 17 mm | Appliance and some ceiling fan fixtures | North America |
| E26 | Medium (standard) | 26 mm | The default household bulb in North America | North America |
| E27 | Medium | 27 mm | The European standard — 1 mm larger than E26 | Europe |
| E39 | Mogul | 39 mm | High-wattage, warehouse and street fixtures | North America |
| E40 | Mogul | 40 mm | European high-wattage equivalent | Europe |
| GU10 | Twist-lock | 10 mm pin spacing | Mains-voltage spotlights | Common |
| GU5.3 | Bi-pin | 5.3 mm pin spacing | MR16 low-voltage spotlights | Common |
| G9 | Loop pin | 9 mm pin spacing | Compact mains-voltage capsules | Common |
Reading the code
E26 means an Edison screw base 26 mm across. GU10 means a
twist-lock pin base with pins 10 mm apart. Once you know that, most codes decode
themselves — the letter is the mechanism, the number is a millimetre measurement.
The pairs that cause problems
- E26 and E27 — 1 mm apart and mechanically compatible. The hazard is electrical, not mechanical: a 120 V bulb in a 240 V fixture is running at twice its rated voltage. Check the voltage marking on the bulb, not whether it screws in.
- E12 and E14 — 2 mm apart. Here the mechanical mismatch is real: an E14 will not seat properly in an E12 holder.
- E39 and E40 — the high-wattage version of the E26/E27 situation, with the same voltage caveat.
A bulb that needs persuading is the wrong base — but a bulb that threads in easily is not automatically the right one. Voltage rating is the check that matters.
Base size is not wattage
A socket's size says nothing about what it can carry — fixture wiring and the fixture's own rating do. LED retrofits made this easy to forget, since a 100 W-equivalent LED in an E26 base draws a fraction of what the old incandescent did. Check the fixture label, not the bulb base.
Data source and method
- Base codes
- IEC 60061 lamp cap designations. The numeric part is the nominal diameter in millimetres by definition.
- Verification
- Cap designations checked against IEC 60061 sheet numbers: E12, E14, E17, E26, E27, E39 and E40 all take their numeric part directly from the nominal thread diameter in millimetres; GU10 and GU5.3 from nominal pin spacing.
- Scope
- Common household and commercial bases. Specialty automotive, projector and stage bases are not covered.
- Not an electrical rating
- Base size describes mechanical fit only. Wattage limits come from the fixture.
Looking for the other kind of socket?
Socket size chart — sockets for bolts and nuts.